Question

Possible Duplicate:
DateTime “null” value

There are many built-in classes in C# that are not Nullable.
For example, Datetime, sometimes I would like to check if my datatime object is null or not but i am not allowed. How can you insert nullability to datetime class ?

Answer 1

Sure you can make an integer, double, datetime, etc. nullable with a change on the declaration.

DateTime? dt = null.

BUT

There are two things you need to be aware of.

1) DateTime? and DateTime are not the same data type so, if you do something like this:

DateTime? date1 = null;
date1 = DateTime.Now;

DateTime date2 = date1; // Wont work.

The correct way of accesing the value of your nullable type is like this:

DateTime date2 = date1.Value;

2) The second BUT is that you need to ALWAYS check if there is a value before you can use it.

DateTime? date1 = null;
DateTime date2 = date1.Value;

This will throw an exception, “Nullable object must have a value”.

So it’s not that simple as adding the nullable operator, you need to check the property “HasValue” every time you want to use your variable:

DateTime? date1 = DateTime.Now;
if(date1.HasValue)
{
    DateTime date2 = date1.Value;
}

Use nullables wisely.